1. Field of the Invention
The present invention relates to a system and method for manufacturing a single-crystal ingot (particularly, a single-crystal silicon ingot) by means of the Czochralski technique (hereinafter referred to as the “CZ technique”), and more particularly, to a system for manufacturing a CZ single-crystal silicon ingot equipped with a cooler which consumes comparatively little power.
2. Background Art
[System for Manufacturing a Single-crystal Ingot by Means of the CZ Technique]
A method of pulling a single-crystal ingot by means of the CZ technique pertains to a known technique, and a system for manufacturing a Czochralski single-crystal ingot has already become widespread. According to the CZ technique, manufacture of a single-crystal ingot involves pulling of a single-crystal ingot from molten raw material. There have already been put forward and recently put into practical use several systems for manufacturing a single-crystal ingot which enables high-speed pulling of a single-crystal ingot by increasing the temperature gradient of the single-crystal ingot in the vicinity of the interface between solid and melt (e.g., Japanese Patent Laid-Open Nos. 256593/1988, 239291/1996, and Japanese Paten No. 2562245/1996).
FIG. 7 is a simplified longitudinal cross-sectional view illustrating an example system of known type which manufactures a single-crystal ingot. As shown in FIG. 7, a system 10 of known type comprises a heat-shielding member 12 which surrounds a single-crystal ingot 11 and shields the heat radiated from the level of molten raw material 15 (hereinafter referred to as a “melt 15”) and a heater 16, and a cooler 13 for cooling a single-crystal ingot which is being pulled (hereinafter called a “single-crystal pulled ingot 11” or called simply “ingot 11”). The cooler 13 is provided for increasing the axial temperature gradient of the single-crystal pulled ingot 11. In order to improve the efficiency of production of a single-crystal ingot by means of increasing the pull rate of the ingot 11, many current CZ single-crystal ingot systems adopt a cooler.
[Tail of a Single-crystal Ingot]
In the manufacture of a single-crystal ingot employing the CZ technique, after a single-crystal ingot has been grown to a desired length, a reversed-conical neck usually called a tail must be formed. If the single-crystal pulled ingot is abruptly pulled out of the melt 15, a crystal dislocation called slip arises in the ingot (a slip-back phenomenon), and the portion where slip is present cannot be used for manufacturing products.
A length along which the slip-back occurs in the ingot is an ingot diameter when the ingot is disconnected from the melt level, thus causing a slip-back phenomenon. In order to maximize the number of wafers which can be produced as products from an ingot, a tail must be produced by means of carefully tapering the diameter of the ingot in the process of reaching the end of pulling operation, so as to avoid causing a slip-back phenomenon in the portion of the ingot to be sliced into wafers (hereinafter referred to as a “body”).
The tail is usually formed to a length equal to the diameter of the body. The reason for this is that if the tail is too short, an area of anomalous oxygen precipitates extends to the body, and the portion of the body where the anomalous oxygen precipitates are present cannot be used for producing wafers. However, the tail cannot be used for producing wafers, and hence an excessively long tail is uneconomical.
A person who is versed in the art well knows that the essential requirement is that a single-crystal ingot shall be pulled to form a tail while the axial temperature gradient of the single-crystal pulled ingot is decreased. To this end, a crucible is usually heated excessively at the time of formation of a tail so as to increase the temperature of the melt, thereby decreasing the temperature gradient of the single-crystal pulled ingot.
However, particularly in a case where the temperature gradient of the single-crystal pulled ingot is intentionally set to be high through use of a cooler, the temperature of the melt must be increased much higher. As a matter of course, such heating of the melt results in an uneconomical increase in power consumption. Further, such heating also induces anomalous heating of a quartz crucible, so that air bubbles existing in the crucible bulge and burst. Broken pieces of the crucible become attached to the single-crystal ingot, thus causing dislocations or rendering the ingot polycrystalline.
In effect, heating the crucible so as to exceed the cooling operation of the cooler requires a large amount of electric power, which in turn requires a larger power unit or involves accelerated deterioration of components provided in the furnace exposed to excessive heat.
In the system of manufacturing a single-crystal ingot, a so-called hot zone of the furnace must be disassembled and cleaned after completion of all the ingot manufacturing processes and removal of a single-crystal ingot from the furnace and before the next processing cycle is started. The hot zone must be sufficiently cooled so that a worker can disassemble the hot zone. Cooling a hot zone of a system of conventional type usually requires six hours or thereabouts, thereby prolonging the per-cycle time required for manufacturing a single-crystal ingot, and deteriorating production efficiency. If the crucible is heated so as to exceed the cooling operation of the cooler, a longer time is required for cooling the hot zone.
The present invention has been conceived to solve the drawbacks in the prior art and is aimed at providing a system for manufacturing a single-crystal ingot which is equipped with a cooler for cooling a single-crystal ingot being pulled and is capable of forming a tail without involvement of excessive heating of a crucible, as well as a method for controlling the system.
As a result of considerable studies in consideration of the foregoing drawbacks, the present inventors have found that a reduction in power consumption and shortening of production time can be achieved, by way of movement of a cooler provided in a single-crystal ingot manufacturing system according to a process of manufacturing a single-crystal ingot, thus completing the present invention.
A reduction in power consumption is primarily achieved by means of moving the cooler away from an interface between a solid and a melt (hereinafter referred to simply as a “solid/melt interface”) during formation of a tail. The present invention has been completed as a result of a discovery that a better result is attained when there is taken into consideration occurrence of defectives, such as crystal grown-in defects due to the re-ascent of the temperature of the single crystal.